Chlorofluorocarbons (CFCs) have been identified as one of the main causes of the depletion of atmospheric ozone. As a result, extensive effort has been directed towards discovering alternatives to CFCs in a variety of applications. In addition to the utilization of CFCs as refrigerants and aerosols, CFCs are commonly used as solvents for the manufacture of fluoropolymers, which are generally insoluble in traditional organic solvents but can often be solubilized in CFCs. Hence, there is a need for alternative methods of manufacturing fluoropolymers.
A supercritical fluid (SCF) is a substance above its critical temperature and critical pressure (or "critical point"). Compressing a gas normally causes a phase separation and the appearance of a separate liquid phase. However, if the fluid is in a supercritical state, compression will only result in density increases: no liquid phase will be formed. The physical properties of supercritical fluids are highly unpredictable, and the use of supercritical fluids for carrying out polymerization processes has received relatively little attention.
S. Kumar et al., Polym. Prep. 27, 224 (1986), describe the free radical precipitation polymerization of polystyrene chains in supercritical ethane. The styrene monomers were dissolved in ethane and polymerized with azobisisobutyronitrile ("AIBN") as an initiator using an apparatus having a high-pressure optical cell.
V. Sarai and E. Kiran, Polym. Prep. 31, 687 (1990), describe the free radical polymerization of styrene in supercritical ethane, propane, and butane, using AIBN, t-butyl peroxide and t-butyl peroxybenzoate as initiators.
K. Scholsky, Polym. Prep. 31, 685 (1990), describe a variety of polymerization reactions using supercritical fluids. The article summarizes publications on the polymerization of fluorinated olefins, such as vinyl fluoride, tetrafluoroethylene, the copolymerization of tetrafluoroethylene and sulfur dioxide, and polyperfluoropropylene. Other fluorinated olefins mentioned include n-tetradecafluoroheptene-1, n-perfluoropentadiene-1,4, vinylidene chlorofluoride, and a variety of vinylic monomers. The polymerization of fluoropolymers in carbon dioxide is not suggested.
E. Beckman and R. Smith, J. Phys. Chem. 94, 345 (1990) describe the microemulsion polymerization of acrylamide (CH.sub.2 CHCONH.sub.2) in supercritical fluids, and particularly the inverse microemulsion polymerization of water-soluble acrylamide monomers within near-critical and supercritical alkane continuous phases.
G. S. Varadarajan, Free Radical Polymerization in Supercritical Fluid Solvents (Nov. 29, 1990) (MIT Doctoral Thesis) describes the free radical polymerization of polymethylmethacrylate (MMA) (CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.3) in supercritical carbon dioxide using AIBN as an initiator. The polymerization of fluoromethacrylates in supercritical carbon dioxide is not suggested.
Canadian Patent No. 1,274,942 is directed to acrylic acid polymerization by homopolymerizing carboxylic acids such as acrylic acid and methacrylic in supercritical carbon dioxide. The polymerization of fluoroacrylate or fluoromethacrylate monomers in supercritical carbon dioxide is not suggested.
V. Krukonis and M. McHugh, Supercritical Fluid Extraction, 156-158, describes the fractionation of a perfluoroalkylpolyether oil and a chlorotrifluoroethylene oligomer in supercritical carbon dioxide, but does not suggest the polymerization of fluoromonomers in supercritical carbon dioxide.